This invention is directed to crystal ovens for maintaining radio frequency control crystals at a predetermined temperature.
By government regulation, radio transmitters are required to hold a precise center frequency, within very strict tolerances over a broad range of ambient temperatures, typically from -20.degree. to 50.degree. C. To meet these requirements, the operating temperature of the crystals used for frequency determination and stabilization in each radio transmitter must be maintained at a predetermined value within close tolerances.
The temperature of the crystals has been controlled through the use of crystal ovens. In broad terms, a crystal oven consists of the following components: (1) a housing, wherein the crystals are mounted in sockets; (2) heater wire wrapped around the exterior of the housing; (3) an oven temperature control circuit that adjustably controls the amount of current flowing through the heater wire as a function of the temperature sensed by the circuit; and (4) an electrical shield enclosing all of the components. The oven temperature control circuit typically includes a temperature sensor (such as a thermistor), a power transistor, resistors and control transistors. The output signal from the temperature sensor causes the oven temperature control circuit to vary current through the power transistor and thus through the heater wire to maintain the temperature of the crystal oven substantially constant.
To work effectively, the crystal oven must be configured to thermally isolate the crystal terminals from ambient temperature. Otherwise, heat will be transferred from the crystals to the environment surrounding the crystal oven by conduction, via electrical leads connected to the crystal socket terminals, and by convection. The heat transfer (in this case, cooling) will, in some environments, prevent maintenance of the crystal temperature within required tolerances.
Another problem associated with heat loss via the crystal socket terminals is that the crystal oven must be constantly and therefore inefficiently heating the crystals to compensate for the heat loss.
The prior art teaches a number of attempts to maximize thermal isolation of the crystals by physically isolating the crystal terminals from ambient temperature. Electrical leads typically connect one terminal of each crystal to a frequency control circuit within the radio but exterior to the crystal oven, and connect the other terminal of each crystal to a common ground. In order to minimize the heat transfer from the interior of the crystal oven to ambient as a result of thermal conduction through the crystal terminals and the associated electrical leads, the prior art discloses the following: the housing for the crystal terminals is positioned away from a thermally insulative board, typically made of Bake-O-Lite; the crystal socket terminals are maintained within the housing; the electrical leads have as small a cross-sectional diameter as possible; and, the electrical leads extend through very small openings in both the housing and the thermally insulative board. To achieve thermal isolation in this manner, prior art crystal ovens have been difficult to assemble and therefore expensive to manufacture. They are also generally unreliable. The small electrical leads extending from the crystal socket terminals are easily broken in shipment or installation, and repairing the broken leads is prohibitively expensive. Additionally, the electrical leads, after they extend through the thermally insulative board, resemble a mass of spaghetti; it is difficult to locate the correct lead from a particular crystal, and to connect that lead, once it is located, to another circuit.
Prior art crystal ovens also lose heat through their electrical shielding. In one prior art crystal oven, a ground plate is mounted to the side of the thermally insulative board opposite the crystal sockets, and is electrically connected to the ground leads coming from the crystal sockets. Since the ground plate is therefore exposed to the environment, its temperature substantially corresponds to ambient temperature and further increases heat transfer from the crystal oven.
In addition to heat generated by the heating wire, the power transistor of the oven temperature control circuit generates a considerable amount of heat which can be used to maintain the temperature of the crystals within tolerances. In some prior art crystal ovens, the collector of the power transistor is mounted on the crystal oven housing to aid in heating the crystals. However, mounting the power transistor on the housing in this manner concentrates a strong heat source at one location so that a temperature gradient is established within the crystal oven. Crystals located adjacent the power transistor may be operating at higher temperatures than crystals located away from the power transistor. Thus, the temperature of some of the crystals may not be maintained within the required tolerances.
Therefore, it is an object of this invention to provide a new and improved crystal oven that thermally isolates the crystal terminals from ambient temperature and is relatively easy and inexpensive to manufacture and maintain.
It is a further object of this invention to provide a new and improved crystal oven that evenly distributes heat throughout the entire crystal oven housing.
It is another object of this invention to provide a new and improved crystal oven that is reliable and that allows specific crystal terminals to be easily identified and easily connectable.
It is yet another object of this invention to provide a new and improved crystal oven which minimizes heat loss from the crystal oven housing through the electrical shielding surrounding the housing.
It is still another object of this invention to provide a new and improved crystal oven that operates more efficiently than prior art crystal ovens.